Rotatable treatment units may be used in water treatment systems. Rotatable treatment units may be employed, for example, to cause oxygen to dissolve in a particular liquid containing contaminants and to enable alternating respiration and submersion of microorganisms, which attach to portions of the rotatable treatment units. Unfortunately, conventional rotatable treatment units are not adjustable or customizable after installation and thus cannot be easily adapted to changing circumstances within the water treatment system. Accordingly, an improved aeration system is desirable.